1. Field of the Invention
The present invention relates to a Group III nitride semiconductor light-emitting device whose light-emitting layer has a MQW (Multi Quantum Well) structure.
2. Background Art
In the conventional Group III nitride semiconductor light-emitting device, a light-emitting layer has a MQW structure to improve light emission efficiency. A structure in which an InGaN well layer and an AlGaN barrier layer are repeatedly deposited has been known as the structure of such light-emitting layer. In this case, the growth temperature of the barrier layer must be higher than that of the well layer to achieve good crystallinity. However, in is evaporated from the well layer due to a temperature increase during the formation of the barrier layer, resulting in deterioration of crystallinity.
Therefore, Japanese Patent Application Laid-Open (kokai) Nos. 2011-35156 and 2011-187862 suggests a structure in which a capping layer is formed at the same growth temperature as for the well layer between the well layer and the barrier layer to prevent In evaporation due to a temperature increase during the formation of the barrier layer. Japanese Patent Application Laid-Open (kokai) No. 2011-35156 discloses that GaN is used as a capping layer, and Japanese Patent Application Laid-Open (kokai) No. 2011-187862 discloses that AlGaN is used as a capping layer. It also discloses that each capping layer has the same Al composition ratio.
In the conventional light-emitting layer, the carrier distribution is uneven and biased on the positive side. This is because the drift velocities of electrons and holes are different: electrons are easy to reach the positive side of the light-emitting layer due to high mobility, and holes are difficult to diffuse to the negative side of the light-emitting layer due to low mobility. Thus, the recombination of electrons and holes is concentrated on the positive side, resulting in a reduction of the light emission efficiency.
To solve this problem, a technique to improve light emission efficiency by reducing the unevenness of the carrier distribution in the light-emitting layer has been studied. However, it is technically difficult to completely eliminate the unevenness of the carrier distribution, and the light emission efficiency cannot be expected to improve much even if the unevenness of the carrier distribution is reduced.